Given a string s representing a valid expression, implement a basic calculator to evaluate it,
 and return the result of the evaluation.

Example 1:

Input: s = "1 + 1"
Output: 2
Example 2:

Input: s = " 2-1 + 2 "
Output: 3
Example 3:

Input: s = "(1+(4+5+2)-3)+(6+8)"
Output: 23

USE A Stack
TC: O(N)
SC: O(N)

class Solution {
public static int calculate(String s) {
	int len = s.length(), sign = 1, result = 0;
	Stack<Integer> stack = new Stack<Integer>();
	for (int i = 0; i < len; i++) {
		if (Character.isDigit(s.charAt(i))) {
			int sum = s.charAt(i) - '0';
			while (i + 1 < len && Character.isDigit(s.charAt(i + 1))) {
				sum = sum * 10 + s.charAt(i + 1) - '0';
				i++;
			}
			result += sum * sign;
		} else if (s.charAt(i) == '+')
			sign = 1;
		else if (s.charAt(i) == '-')
			sign = -1;
		else if (s.charAt(i) == '(') {
			stack.push(result);
			stack.push(sign);
			result = 0;
			sign = 1;
		} else if (s.charAt(i) == ')') {
			result = result * stack.pop() + stack.pop();
		}

	}
	return result;
}
}